JSON Schema requires two fields to include the $ prefix.

First is the $schema keyword that defines the version and URI of the schema to be stored.

Second is the $ref used to describe pointers to additional schema data.

More info on JSON Schema is available here: http://json-schema.org/documentation.html

As JSON Schema is becoming a more heavily used tool for JSON data management

As JSON is requested for config file support SERVER-16819
And JSON Schema validation as well SERVER-3536

I see it as a useful exception to the no $ prefix error and it could even be useful to allow $ref resolution and caching in future (as a separate issue).